Crystal-controlled mechanical resonator

ABSTRACT

A MINIATURE TIMEPIECE DRIVEN BY A TUNING-FORK MOTOR WHOSE VIBRATORY ACTION IS ELECTROMAGNETICALLY SUSTAINED BY AN ELECTRONIC DRIVE CIRCUIT. TO REGULATE THE TIMING RATE OF THE MOTOR, A FREQUENCY STANDARD IS PROVIDED IN THE FORM OF A CRYSTAL OSCILLATOR WHOSE FREQUENCY IS AN EXACT MULTIPLE OF THE PRESCRIBED TIMING FREQUENCY OF THE MOTOR. THE MOTOR IS PRESET TO OPERATE AT A TIMING RATE DISPLACED TO ONE SIDE OF THE PRESCRIBED FREQUENCY TO A DEGREE DETERMINED BY THE MAXIMUM ANTICIPATED DEVIATION IN TIMING RATE CHARACTERISTIC OF THE MOTOR. TO EFFECT REGULATION, PULSES ARE DERIVED FROM THE ELECTRONIC CIRCUIT, THE PULSES HAVING A PERIODICALLY EQUAL TO THE OPERATING FREQUENCY OF THE FORK. THESE PULSE ARE COMPARED IN A COINCIDENCE CIRCUIT WITH THE STANDARD OSCILLATIONS TO PRODUCE AN ERROR SIGNAL ONLY WHEN THE PULSE ARE DISPLACED IN PHASE FROM THE PARTICULAR SUBMULTIPLE OF OSCILLATIONS WHICH CORRESPONDS TO THE PRESCRIBED FREQUENCY. THE ERROR SIGNAL ACTIVATES A FREQUENCY-CORRECTION CIRCUIT COUPLED TO THE ELECTRONIC DRIVE CIRCUIT ASSOCIATED WITH THE TUNING-FORK MOTOR, WHEREBY THE PHASE OF THE PULSE DERIVED FROM THE ELECTRONIC CIRCUIT SHIFTS IN A DIRECTION TO BRING ABOUT PHASE-LOCKING WITH SAID SUBMULTIPLE.

NOV. 2, W .O. BENNETT ETAL CRYSTAL-CONTROLLED MECHANICAL RESONATOR Filed March 19, 1970 7795 D/spuv PULSE A* Iv I.. l-

ffm-f @www INV/Mums HY @auf /Q X05/f2 @mevr/ov ffvefasms] W w- Armen/5y United States Patent O 3,616,638 CRYSTAL-CONTROLLED MECHANICAL RESONATOR William 0. Bennett, Bayside, N.Y., and Dale R. Koehler, Westwood, NJ., assignors to Bulova Watch Company,

Inc., New York, N.Y.

Filed Mar. 19, 1970, Ser. No. 21,033 Int. Cl. G04c 3/00 U.S. Cl. 58--23 TF 10 Claims ABSTRACT F THE DISCLOSURE A miniature timepiece driven by a tuning-fork motor whose vibratory action is electromagnetically sustained by an electronic drive circuit. To regulate the timing rate of the motor, a frequency standard is provided in the form of a crystal oscillator whose frequency is an exact multiple of the prescribed timing frequency of the motor. The motor is preset to operate at a timing rate displaced to one side of the prescribed frequency to a degree deter mined by the maximum anticipated deviation in timing rate characteristic of the motor. To effect regulation, pulses are derived from the electronic circuit, the pulses having a periodicity equal to the operating frequency of the fork. These pulses are compared in a coincidence circuit with the standard oscillations to produce an error signal only when the pulses are displaced in phase from the particular submultiple of oscillations which corresponds to the prescribed frequency. The error signal activates a frequency-correction circuit coupled to the electronic drive circuit associated with the tuning-fork motor, whereby the phase of the pulses derived from the electronic circuit shifts in a direction to bring about phase-locking with said submultiple.

BACKGROUND OF INVENTION This invention relates generally to crystal-controlled mechanical resonators, and more particularly to a batteryoperated timepiece of miniature size in which the operation of a vibratory motor is phase-locked to a crystal oscillator standard of much higher frequency.

In order to provide timepieces of exceptional accuracy, it has heretofore been known to use a stable piezoelectric crystal oscillator as a frequency standard, timing pulses derived from the oscillator serving to actuate or control a vibratory or synchronous motor for driving the gear works of a timepiece. Because the lowest operating frequency of a crystal oscillator feasible for this purpose lies in the kilohertz range, it is necessary to reduce this frequency to a much lower value. This ordinarily is accomplished by a chan of divider stages constituted by binary, flip-flop or multivibrator circuits which yield output pulses whose rate is an exact submultiple of the crystal frequency.

Thus, in the patent to Krassoevitch et al. 2,976,470, there is disclosed a timepiece in which a small synchronous timing motor is powered by low-frequency pulses derived from a high-frequency crystal oscillator, the frequency of the oscillator being divided down to less than 50 Hz. The Schaller Pat. 3,282,042 makes use of low-frequency pulses derived by division from a high-frequency crystal oscillator to synchronize the operation of a tuning fork resonator which drives the gear works of a timepiece through a pawl and ratchet mechanism.

In the context of battery-operated watches and other timepieces of miniature size, there are two factors which are of primary importance in scaling down a crystal-controlled system so that it can be accommodated within the contines of a small casing. The first and obvious factor is the size of the components forming the system. The state of the art is such that crystal oscillators and frequency ACC dividers, by means of solid-state, microelectronic techniques, may be fabricated in dimensions compatible with watch requirements. lt is also possible to provide highly compact vibratory or stepping motors suitable for watch purposes.

The second factor is, however, much more troublesome, for this factor deals with the electrical power requirements of the system. It is not feasible within the confines of a battery-operated watch to provide more than a single, disc-shaped dry cell having a voltage of, say, 1.3 or 1.5 volts. The capacity of such cells is very limited; hence, an operating current drain in the milliampere range would exhaust the cell in a relatively brief period. In order to obtain a battery life of a year or more, it is essential that the average operating current drain not exceed about ten microamperes.

Presently-known low-voltage solid-state frequency-divider stages for reducing the frequency of a crystal oscillator whose resonant frequency is ten kilohertz or higher to a relatively low submultiple thereof (i.e., one to three hundred Hz,) entail a large number of transistors or other solid-state elements and associated components. Hence the current requirements for the circuit are relatively high. In an attempt to reduce power consumption as well as to cut down on the number of components involved, it has recently been proposed that the phase-lock technique be used in lieu of frequency-division to crystal-control the operation of a mechanical resonator for driving time indicators.

A standard phase-lock loop contains three basic components, namely a phase detector, a low-pass filter and a voltage-controlled local oscillator whose frequency is controlled by an external signal from a reference oscillator. The phase detector compares the phase of a periodic reference signal against the phase of the voltage-controlled oscillator to provide an output which reflects the phase difference between the two inputs. The difference voltage is then filtered by the loop filter and applied to the local oscillator. The control voltage on the local oscillator changes the frequency thereof in a direction that reduces the phase difference between the input signal and the local oscillator. To maintain the control voltage needed for locking, it is generally necessary to have a non-zero output for the phase detector.

In adapting the phase-lock technique to a timepiece having a low-frequency resonator motor which is to be operated at a submultiple of the resonance frequency of a stable crystal reference oscillator, the phase detector acts to sample the output of the reference oscillator so that, assuming a ratio of 20:1 between the reference and local oscillator frequencies, every 20th cycle of the reference frequency will be sampled to pro-vide a control voltage which depends on the phase displacement. A more detailed description of the phase-locking technique and circuits appropriate thereto may be found in the 1966 text, Phaselock Techniques, by Floyd M. Gardner (John Wiley and Sons).

Because a solid-state phase-lock system, as compared to a multi-stage frequency divider, involves a smaller number of solid-state components and demands less current to operate, it would appear to halve distinct practical advantages in the context of a miniature timepiece where space and power are at a premium. But since, in this context, the local oscillator is a mechanical resonator, the power input of which is affected by phase-lock operation, this factor must be taken into account in considering the current requirements of the system.

Resonance is the state attained by a mechanical oscillator such as a pendulum, a vibrating reed, a torsional spring or a tuning fork when it is driven by an oscillating external force whose frequency approximates the systems own natural frequency. In a mechanical resonator, the mass vibrates or oscillates about an equilibrium position.

Let us now consider a mechanical resonator being driven by an external force whose frequency is equal or is almost equal to the natural frequency thereof, for example, a pendulum that is lightly struck at the beginning of each period. It receives an increment of energy at every stroke, causing it to oscillate with increasing amplitude. Neglecting friction, its amplitude of oscillation would increase indefinitely. In practice, however, the amplitude increases only until the energy acquired from the driving force per cycle exactly balances the losses incurred during each cycle.

A useful figure of merit of a mechanical resonator is the ratio of total energy of oscillation to the energy lost per cycle, called the Q of the system. A resonance curve may be produced by plotting the steadystate amplitude against the driving force frequency for constant power input to the resonator. The higher the Q of the mechanical resonator, the sharper is the resonance about the natural frequency.

Where a mechanical resonator, such a vibratory reed, a torsional pendulum or a tuning fork, is electrically actuated and is used as a motor to drive the gear works of a timepiece in which the vibratory or oscillatory motion is converted into rotary motion, the Q of the resonator is an important factor. This is because the higher the Q, the less electrical energy is required to attain the desired operating amplitude at the resonance frequency, all other factors (size, frequency, amplitude, etc.) being substantially the same.

The Q of a crystal oscillator is usually in the ten thousands range and higher, that of a tuning fork in the thousands range, and that of a reed or torsional vibrator in the hundreds range. Hence, as between a tuning-fork motor or a torsional-vibrator motor of about the same size, much less current is required to operate the tuning-fork motor, the difference being equal to the ratio between Qs, other factors being equal.

-In the envisioned phase-loc-k system, the sampled output of a high-frequency crystal is compared with electrical signals derived from an electromagnetically-actuated mechanical resonator. Any phase displacement between the input signals is sensed by a comparator circuit which develops an error signal to cause a frequency change in the resonator, thereby to effect phase correction.

A low-Q vibratory motor such as one involving a torsional pendulum, or a vibrating reed, when controlled by a phase-lock loop system, because it has a relatively broad resonance curve, has the advantage of being almost unaffected by the requirements of the system. The reason for this is that when the frequency of the low-Q vibrator is shifted by the system away from its resonance point, the amplitude thereof is not significantly reduced, and the current drawn by the resonator drive circuit is only slightly increased. For this reason, one might conclude that the low-Q vibratory motor is to be preferred over a high-Q slaved resonant system for driving the time-indicating hands of a watch or other miniature timepiece. The fact is, however, that while the percentage decrease in amplitude and the concomitant increase in power to drive the low-Q resonant system a given amount off-frequency is considerably less than for a high-Q resonator, the total driving power is considerably greater for the low-Q system. This is one of the main reasons that the high-Q system is advantageous in the context of a battery-operated watch.

SUMMARY OF INVENTION In view of the foregoing, it is the primary object of the present invention to provide a battery-operated timepiece of miniature size in which a high-Q tuning-fork motor or any other form of high-Q mechanical resonator is phase-locked to a crystal oscillator of much higher frequency.

More particularly, it is an object of the invention to provide a timepiece of exceptional accuracy and efficiency which is capable of being housed in a relatively small watch casing, the current requirements of the crystalcontrolled, phase-locked tuning-fork system being so small as to make possible an extended operating life even with a miniature single-cell battery of very limited capacity.

Among the significant features of the invention are the following:

"(A) The use of a solid-state, phase-lock control system in conjunction with a crystal oscillator reduces the number of transistors required as compared to a frequency-divider, and demands less current to operate.

(B) Even though the tuning-fork motor is a high-Q device, and is subject to detuning by the phase-lock circuit, thereby causing an increase in operating current, the operating arrangement is such that the overall current requirements of the motor are far smaller than those of a comparable phase-locked, low-Q resonant motor of equivalent frequency, size and other power-related factors.

(C) The use of a high-Q tuning-fork motor makes it possible to effect phase-locking unilaterally rather than bilaterally; that is, frequency-correction only with respect to a deviation on one side of the prescribed frequency, thereby simplifying the phase-lock system.

(D) The tuning-fork motor is adapted to operate the timepiece with a fair degree of accuracy even upon failure of the phase-lock system or the associated crystal oscillator so that the timepiece continues to function should any defect arise therein other than in the motor.

Also an object of the invention is to provide a compact, crystal-controlled watch which may be manufactured at relatively low cost.

Briey stated, these objects are accomplished in a battery-operated timepiece wherein the same battery, preferably in the form of a single cell, acts to power both a tuning-fork motor and a high-frequency crystal oscillator, the motor including a high-Q tuning-fork resonator whose vibrations are electromagnetically sustained by an electronic circuit including a drive coil. The operating frequency of the crystal oscillator is an exact multiple of the timing frequency prescribed for precise time indications. The motor, however, is preset to operate at a timing rate displaced to one side of the prescribed frequency to a degree determined by the maximum anticipated deviation in timing rate characteristic of the motor in seconds per day.

To effect regulation of the motor, pulses derived from the electronic circuit and having a frequency equal to the vibratory rate of the fork are compared in a coincidence circuit with oscillations derived from the crystal, to produce an error signal only when the pulses are displaced in phase from the particular submultiple oscillations which correspond to the prescribed frequency.

The error signal activates a frequency-changing circuit adapted to shunt a capacitor across the tuning fork drive coils, causing a shift in the frequency of the tuning fork in the direction to make its rate slower, the amount of this shift depending upon the value of the capacitor. Although the frequency-changing circuit can cause only a uni-directional change in the operating frequency of the tuning fork from its natural frequency (in this instance, slower), lai-directional adjustment of the tuning-fork frequency by means of the present system is nevertheless possible within a pre-determined plus and minus rate range from the prescribed frequency. Also, the range for plus and minus corrections need not necessarily be symmetrically disposed in respect to the prescribed frequency.

OUTLINE OF DRAWING For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic circuit diagram of a preferred embodiment of a crystal-controlled, phase-locked tuning-fork motor for driving a timepiece in a system in accordance with the invention;

FIG. 2 is a graph showing the resonance curves of low-Q, medium-Q and high-Q resonators;

FIGS. 3A, B and C show various wave forms produced in the system; and

FIG. 4 illustrates 0n a frequency scale the various rates involved in the operation of the tuning-fork motor.

DESCRIPTION OF INVENTION Referring now to FIG. 1, there is shown a crystal-controlled, phase-locked tuning-fork motor for a miniature timepiece. The principal stages of the system are a vibratory motor constituted by a high-Q mechanical resonator 1,0, sustained in vibration at its natural rate by an electronic drive circuit 11, the operation of the motor being phase-locked with a piezoelectric crystal oscillator, generally designated by numeral 12, by means of a pulseforming circuit 13, a coincidence circuit 14 and a frequency-changing circuit 15. The entire system is powered by a single battery cell 16.

lThe frequency of the crystal oscillator is harmonically related to and is an exact multiple of the prescribed freqltifency of the tuning-fork motor. By prescribed frequency is meant that vibratory rate at which the time display indicators associated with the motor present the tiine precisely. By way of example, we shall assume that the prescribed frequency is 360 Hz., and that the freqbency of. the crystal oscillator is 11,520 Hz., in which event the crystal frequency yis the thirty-second multiple of the frequency of the mechanical resonator. Otherwise stated, the prescribed frequency is the thirty-second submultiple of the crystal frequency. It is to be understood that these frequencies are chosen merely as examples and that, in practice, other ratios may be used, such as 12,000 Hz. and 300 Hz.

/FCrystal-controlled oscillator 12 may be of any known transistorized type, such as those employing the circuits described in' Transistor Electronics (1955), published by Prentice-Hall, Inc. In the circuit shown in FIG. 1, the oscillator includes a transistor 17 and a piezoelectric crystal 18 in a suitable holder.

The Q of a crystal-controlled oscillator is very high (much higher than a tuning fork) and its frequency is very stable. To reduce the sensitivity of the oscillator to V,temperature variations, thermal compensation may be effected by means of compensating networks including bimetallic capacitors, such as capacitor 19, or temperatureresponsive diodes or inductors which are adapted to shift the frequency of the crystal circuit as a function of temperature to correct for changes therein.

The ultimate precision of the timepiece depends on the stability of the crystal oscillator; hence, for extremely ,high orders of precision, the oscillator frequency must be independent of temperature changes. However, even when temperature compensation is not fully effective, the system is still exceptionally accurate as compared to a timepiece having a tuning-fork motor without crystal control.

The tuning-fork motor `is of the type disclosed in U.S. Pat. Re. 26,322 of Bennett et tal., wherein the vibratory motion of la tuning fork is sustained by a battery-powered, transistorized drive circuit, the fork vibrations being converted into rotary motion by means of a ratchet-andpawl mechanism. The pawl engages and advances a ratchet wheel provided with a pinion for operating the timepiece hands through a train of gears. In practice, other forms of motion-converters may be used, such as magnetic escapements.

In the prior patent to Bennett et al., Re. 26,209, there is disclosed a frequency regulator for a tuning-fork motor making possible a final and precise adjustment of the timing frequency. This regulator takes the form of a small, unbalanced loading mass in clip form, pivotally secured to and adjacent the free end of each tine of the fork, whereby rotational displacement of the clip about its pivot serves to shift the center of gravity of the associated tine. With a regulator of this type, it is possible to adjust the resonant frequency of the tuning fork of this invention to the exact value chosen for optimum operation of the phase-lock system.

Mechanical resonator 10 is a tuning fork having a pair of tines 10A and 10B joined together by a base 10C and provided with an upwardly-extending stem 10D secured to the pillar plate of the Watch by suitable screws. The natural frequency of the fork is determined by its geometry, and is adjustable by clip-type or other forms of regulators.

The tuning fork is actuated by electromagntic transducers constituted by permanent magnets 20 and 21 supported on the ends of tines 10A and 10B, respectively. Magnet 20 cooperates with a stationary, phase-sensing coil 22 in the electronic drive circuit, while magnet 21 cooperates with a stationary drive coil 23. In practice, drive coil 23 may be divided into two serially-connected sections, one of which cooperates with magnet 20 rather than with magnet 21, so that both tines rather than one are driven.

The tuning fork drive circuit includes a transistor 24 whose base is coupled to sensing coil 22 through a resistance-capacitance bias network 25, 26, the collectoremitter circuit of the transistor being coupled to drive coil 23. The single-cell battery 16 which is connected to the drive coil through the transistor circuit is of the constant-voltage type, such as a single mercury cell providing a steady voltage (i.e., 1.3 volts) for almost the full duration of its usable life. The same battery, as pointed out previously, is the sole source of power not only for the electronic drive circuit but also for all other stages of the system.

In operation, a pulse applied to drive coil 23 will cause an axial thrust on the associated permanent magnet element in a direction determined by the polarity of the pulse in relation to the polarization of the magnet, to an extent depending on the energy of the pulse. Since the magnet is attached to a tine, the thrust thereon acts mechanically to excite the fork into vibration.

The resultant movement of the magnets relative to the fixed sensing and drive coils induces a back EMF therein, which back EMF takes the form of an alternative voltage whose frequency corresponds to the fork rate. The back EMF` voltage induced in phase-sensing coil 22 is applied to the base of transistor 24 to control the instant during each cycle when the driving pulse is to be delivered to the drive coil, whereas the back EMF voltage induced in the drive coil serves to control the amplitude of the drive pulses. The behavior of the electronic drive circuit is explained in greater detail in the above-identified Bennett patents.

The interaction of the electronic drive circuit and the tuning-fork resonator is self-regulating and functions not only to cause the tines to operate at their natural frequency, but also to maintain these oscillators at a substantially constant amplitude. Attached to tine 10B is a pawl or drive finger 27 which engages a ratchet wheel 28, whereby the vibratory motion of the pawl is converted into rotary motion for driving a gear train 29. Gear train 29 operates the usual hour, minute and second time-indicating hands 30.

The tuning-fork motor is a self-suicient timing device of high accuracy. It is rendered even more precise by phase-locking the operation thereof with a submultiple of the oscillations of crystal standard 12. To accomplish this result, a sinusoidal wave, represented by wave form 31 and reflecting the vibratory action of the tuning fork, is taken from phase-sensing coil 22 and fed to pulse-forming circuit 13, which functions to shape the cyclical wave into relatively sharp pulses whose frequency corresponds to that of the voltage developed across the phase-sensing coil.

Pulse-forming circuit 13 may be constituted by saturated amplifiers and suitable networks to yield a pulse 32 for each cycle of voltage induced in the sensing coil by the vibrating tine associated therewith. Pulses 32 at the output of the pulse-forming circuit 13 therefore represent the operating frequency of the tuning fork motor and they are applied to one input of coincidence circuit 14. This circuit may be any known form of solid-state AND- type arrangement which produces an output only when the inputs thereto coincide in time.

Applied as another input to coincidence circuit 14 is the high-frequency voltage indicated by wave form 33, derived from the crystal oscillator through an R-C isolation network 34, such that an output is produced only when the pulses 31 are coincident with the particular submultiple of the crystal oscillations having the corresponding rate. Thus the coincidence circuit effectively samples the crystal oscillations, for no output is produced except with respect to the .particular submultiple of interest.

The relationship between the wave forms is shown in FIG. 3, where it will be seen that the crystal oscillation frequency '33 is a multiple of the fork frequency 31, and that pulses 32 derived from the fork oscillations coincide with a submultiple of the crystal frequency.

The output of coincidence circuit 14 is applied to the base of a switching transistor 35 in frequency-changing circuit 15, the transistor being connected in series with a frequency-shifting capacitor 36 across drive coil 23. Thus, when switchingtransistor 35 is rendered conductive, capacitor 26 is effectively shunted across the drive coil to shift the resonator frequency. The arrangement is such that transistor 35 is rendered conductive only in the absence of a coincident signal from circuit 14, so that the capacitor is shunted across the drive coil when pulses 32, taken from the tuning-fork electronic drive circuit, are displaced in phase with respect to the prescribed submultiple of the oscillations taken from the crystal oscillator.

As shown in FIG. 2, with a high-Q resonator, the resonance wave, as indicated by curve X, has a relatively sharp peak at the resonance frequency. The amplitude of oscillation drops off sharply on either side of the resonance peak. With a medium-Q resonator, the drop-off in amplitude is much less sharp as one shifts in either direction away from resonance, whereas, with a low-Q resonator, the drop-off in amplitude is still less pronounced.

Since the tuning fork is a high-Q resonator whose Q 1s much higher than most other mechanical resonators, such as vibrating reeds or torsional pendulums, the normal effect of a phase-lock system adapted to displace the frequency of the tuning fork away from the natural resonance frequency thereof will be to render the system less eflicient and to cause a marked increase in current consumption. On the other hand, if the frequency-shift introduced by the phase-lock circuit is extremely small, so that the fork continues to operate very close to its resonance peak, the adverse effect thereof on amplitude and current drain will be minimal.

The Q of a mechanical resonator represents the ratio of energy stored per cycle to energy dissipated per cycle in the oscillator. The formula for rate change in seconds per day, as a function of the phase-shifting of the feedback loop in a self-sustaining oscillator, is:

750. o At Q AA Where A is the electrical phase angle in degrees. This equation shows that a high Q gives rise to a small rate change and therefore provides good stability.

With a resonator which is inherently unstable, in using the phase-lock technique one must be prepared to effect correction bilaterally, for the frequency of the resonator may deviate from the preset frequency in either direction, by fairly large amounts. But with a stable high-Q resonator, such deviations are small and predictable, making possible a less complicated unilateral correction system.

To accomplish bi-directional adjustment of the tuning fork frequency from the prescribed frequency, by means of unilateral phase-locking, the range of frequency correction must first be established, in each direction. This range depends largely upon the characteristics of the tuning-fork motor. For example, the maximum rate in the fast direction which can be encountered in practice is the arithmetic sum of the maximum rate change in the fast direction as a result of the maximum temperature change; the rate change resulting from operation on a mountaintop at, say, 10,000 feet elevation; the maximum rate change in the fast direction which can occur as a result of change in position (attitude) of the tuning fork; and the maximum rate change expected as a result of long-time use, including changes in movement friction, etc. Such changes might total, for example, ten seconds per day. The maximum rate in the slow direction may be similarly established by combining all factors which can contribute to a slow tuning-fork rate in long-time normal use. Such factors might, for example, result in a maximum expected rate in the slow direction of eight seconds per day.

The envisioned phase-lock system can be readily designed to correct for the tuning fork within the rate range from ten seconds per day gaining, to eight seconds per day losing, cited as an example. In this instance, the resonant frequency of the tuning fork is adjusted so that, in the absence of phase-lock frequency correction, the timepiece will gain eight seconds per day. Thereafter, the value of the capacitor, which is shunted across the tuning fork drive coils when the frequency-changing circuit is operating, may be chosen. In the example cited, this capacitor would be so chosen as to change the tuning-fork frequency eighteen seconds per day (slower). The natural frequency of the tuning fork in this example causes a gaining rate for the timepiece of eight seconds per day, and the frequency-correction circuit, at described, changes the frequency in the slower direction (when operating) by eighteen seconds per day. Thus, it is possible for the phaselock system to keep the tuning-fork frequency at the prescribed value, even though the resonant frequency of the tuning fork should deviate from the preset value sufficiently to cause a rate change as much as ten seconds per day faster, or eight seconds per day slower than its nominal uncorrected rate (frequency).

Thus, as shown graphically in FIG. 4, frequency F1 is the prescribed frequency of the tuning fork, this being the vibratory rate at which the tuning fork produces precise time indications, assuming absolute stability. But the fork is actually subject to deviations, so that in accordance with the invention, the fork is preset to frequency F2, which is the frequency to which the fork is adjusted to cause it, in the absence of phase-lock frequency correction, to gain eight seconds a day.

'Since the tuning fork characteristic is such, in the example given, that it will gain a maximum of ten seconds per day and lose a maximum of eight seconds per day, with the fork preset at frequency F2 to have a gain of eight seconds per day, in operation this frequency may increase above Fz by ten seconds per day to reach the maximum uncorrected frequency F3, or it may go down eight seconds to reach the prescribed frequency Fx. The spread therefore between F1 and F3 is eighteen seconds per day fast, whereas the effect of the frequency-correction capacitor is to change the fork frequency, so that it is eighteen seconds per day slower. At no time does the fork frequency go below the prescribed frequency F1, so that the operation of the correction system is unidirectional.

A crystal-controlled high-Q vibratory resonator in accordance with the invention is not only usable for timekeeping purposes in which the resonator acts as a vibratory motor, but may also be used as a stabilized lowfrequency generator serving as a frequency standard or for any other purpose. Also in lieu of timekeeping, the stabilized high-Q resonator may be utilized as a constantspeed motor which is battery-operated. Moreover, the invention is not limited to tuning-fork resonators, and the resonator whose frequency is phase-locked may be any other known form of high-Q resonator whose vibrations are sustained by an electromagnetic drive system, such as the mechanical resonators disclosed in the Dietsch Pat. 3,349,305 and in the Allison Pat. 3,150,337.

While there has been shown and described a preferred embodiment of a crystalcontrolled tuning-fork timing motor in accordance with the invention, it will be appreciated that changes and modifications may be made therein -without departing from the essential spirit of the invention.

We claim:

1. A system for stabilizing the operation of a lowfrequency mechanical resonator to produce a vibratory motion at a prescribed frequency, said system comprising:

(A) a low-frequency resonator constituted by a vibrating element having a relatively high Q, and an electronic drive circuit including an electromagnetic drive coil operatively coupled to said element to sustain the vibratory motion thereof,

(B) means to preset the operating frequency of said resonator so that it is displaced to one side of said prescribed frequency, said resonator being subject to frequency deviation on either side of the preset frequency, the displacement between said prescribed and said preset frequency being substantially equal to the maximum deviation in the direction of said prescribed frequency whereby, in the absence of correction, the operating frequency will not go to the other side of said prescribed frequency while it may reach an uncorrected frequency determined by the maximum deviation in the direction away from said prescribed frequency,

(C) a crystal-controlled generator having a frequency which is a multiple of said prescribed frequency,

(D) means to compare pulses derived from the electronic circuit and appearing at a rate determined by the operating frequency of the resonator with oscillations derived from said generator to produce an error signal only when the pulses are displaced in phase from the particular submultiple of said oscillations which corresponds to said prescribed frequency, and

(E) a frequency-correction circuit coupled to said electronic circuit and responsive to said error signal to shift the operating frequency of the resonator in a direction effecting phase-locking with said submultiple.

2. A system as set forth in claim 1, wherein said element is a tuning fork.

3. A system as set forth in claim 2, further including means to convert the vibratory action of said tuning fork into rotary motion.

4. A system as set forth in claim 3, further including means to transmit said rotary motion to time indicators.

5. A system as set forth in claim 1, wherein said element is a cruciform vibrator.

6. A system as set forth in claim `1, wherein said means to compare said pulses and said oscillations is constituted by a coincidence circuit.

7. A system as set forth in claim 1, wherein said frequency-correction circuit includes acapacitor which is shunted across said coil and has a value capable of causing the operating frequency of the resonator to shift from said uncorrected frequency to said prescribed frequency.

8. A system as set forth in claim 1, wherein said crystal-controlled generator includes means to compensate for temperature effects.

9. A miniature, battery-operated timepiece comprising:

(A) a mechanical resonator constituted by a tuning fork and a battery-operated electronic drive circuit therefor to sustain the vibratory motion of the fork,

(B) means to convert said vibratory motion into rotary motion for operating the time indicators of said timepiece,

(C) means to preset the operating frequency of said resonator so that it is displaced to one side of a prescribed frequency at which said indicators display the precise time, said resonator being subject to frequency deviation on either side of the preset frequency, the displacement between said preset and said prescribed frequency being substantially equal to the maximum 'deviation in the direction of said prescribed frequency whereby, in the absence of correction, the operating frequency will not go to the other side of said prescribed frequency while it may reach an uncorrected frequency determined by the maximum deviation in the direction away from the prescribed frequency,

(D) a battery-operated crystal-controlled generator having a frequency which is a multiple of said prescribed frequency,

(E) means to compare pulses derived from the electronic circuit and appearing at a rate determined by the operating frequency of the resonator with oscillations derived from said generator to produce an error signal when the pulses are displaced in phase from the particular submultiple of the oscillations which corresponds to said prescribed frequency, and

(F) a frequency-correction circuit coupled to said electronic circuit and responsive to said error signal to shift the operating frequency in a direction effecting phase-locking with said submultiple.

10. A timepiece as set forth in claim 9, wherein a single battery cell energizes both the electronic circuit and the generator.

References Cited UNITED STATES PATENTS 3,282,042 11/ 1966 Schaller 58v-23 TP 3,451,210 6/ 1969 I-lelterline, Ir., et al. 58--24 RICHARD B. WILKINSON, Primary Examiner E. C. SIMMONS, Assistant Examiner U.S. Cl. X.R. 331-25, 156 

